An implant of the aforementioned type is described in the DE 42 30 118 C2. This implant can replace at least a portion of the condyle surface of a femur bone. For this purpose, it comprises a femur cap which is mounted on the bone by means of a pin. The femur cup is seated on a prepared plateau surface of the bone.
In order to guarantee a secure hold of the implant on the bone, it must be ensured that the bone cement used for fixing the implant produces an as large as possible joining surface and enters into all cavities. To ensure this whilst only lightly pressing on the implant and to prevent bone cement or a flowing bone material from laterally oozing out over the cup, the latter is at its edge provided with a continuous circularly peripheral, archlike concave shape.
Other similar implants are described in EP 0 709 075 A1, EP 0 611 559 A1 and WO 95/24874.
However, all of the aforementioned implants do not solve the following problem. When a joint is stretched, the loaded joint seating surface between the implant and the associated joint base should be as large as possible, thus ensuring small surface pressure and good stability. Small surface pressure prevents, amongst other things, rapid wear of the artificial joint section.
A large seating surface is obtained in that a large curvature radius of the joint surface, or a small curvature, is selected in this area.
However, the desired large surface seating with a stretched joint offers the following disadvantage. If the implant is inserted transversely in such a manner that its longitudinal alignment does not coincide with the pivotal plane of the joint, then edge pressure may develop when bending the joint. This edge pressure makes excessive demands on both the joint base and the implant itself and causes rapid wear.